title: Interhemispheric balance in Parkinson's disease: a transcranial magnetic stimulation study.
abstract: BACKGROUND
  Parkinson's disease (PD) is characterized by various changes in motor excitability.
  
  
  OBJECTIVE
  To examine through Transcranial Magnetic Stimulation (TMS) cortical excitability, specifically addressing interhemispheric connections in PD.
  
  
  METHODS
  Nineteen PD patients with a predominant involvement of the left hemibody (7 females, age 61.7 years,) and 13 controls (6 females, age 61.5 years) entered the study. Patients were subdivided into two groups (early and advanced) according to the time from PD diagnosis. Participants underwent evaluation of Resting Motor Threshold (RMT) and ipsilateral Silent Period (iSP), induced by suprathreshold TMS on the ipsilateral-M1, measured as suppression of voluntary EMG activity. Mirror Movements (MM) were EMG-recorded and scored, in three upper limb muscles, during unilateral voluntary hand movement. Patients were studied at baseline (OFF drug) and after acute levodopa challenge (ON).
  
  
  RESULTS
  PD patients showed a general reduction in RMT vs controls (P < 0.01 for right and left hemisphere) in both drug conditions. Early PD had a significantly lower RMT over the right vs the left hemisphere (P = 0.027); this difference was no longer significant after levodopa. In early PD patients, MM were mainly observed in the right arm during voluntary activation of the left, more affected side both in OFF (P = 0.033) and in ON (P = 0.046). In PD, RMT of the left, less affected M1 was significantly correlated with the right lateralized motor score (P = 0.011; Spearman's coefficient = -0.585), as well as with disease duration. In PD patients, a shorter (P = 0.039) and smaller (P = 0.037) iSP was detected when the stimulus was applied to the worse M1 (right) compared with the contralateral side. This asymmetry was significant only OFF drug. In the PD group iSP-duration from the right, less affected APB was negatively correlated with the MM recorded from the same side during the voluntary movement of the worse side (Spearman's coefficient = -0.498; P = 0.035).
  
  
  CONCLUSIONS
  Increased cortical motor excitability in PD, consistent with previous findings, is more evident in the worse hemisphere, particularly in early PD. Asymmetric motor involvement is also associated with excessive involuntary mirroring and defective interhemispheric inhibition, both unfavoring the more affected side. Altogether, these findings suggest that asymmetric motor involvement in PD, particularly in the earlier phases of the disease, affects the interhemispheric balance of cortical excitability, movement lateralization and transcallosal inhibition.
authors.affiliation: Neurological Department, Institute of Experimental Neurology (INSPE), San Raffaele Hospital, Milan, Italy; Experimental Neurophysiology Unit, Institute of Experimental Neurology (INSPE), San Raffaele Hospital, via Olgettina 60, I-20132 Milan, Italy; Vita-Salute University San Raffaele, Milan, Italy. Electronic address: francesca.spagnolo@hsr.it.
authors.forename: Francesca
authors.initials: F
authors.lastname: Spagnolo
authors.forename: Elisabetta
authors.initials: E
authors.lastname: Coppi
authors.forename: Raffaella
authors.initials: R
authors.lastname: Chieffo
authors.forename: Laura
authors.initials: L
authors.lastname: Straffi
authors.forename: Mario
authors.initials: M
authors.lastname: Fichera
authors.forename: Arturo
authors.initials: A
authors.lastname: Nuara
authors.forename: Javier
authors.initials: J
authors.lastname: Gonzalez-Rosa
authors.forename: Vittorio
authors.initials: V
authors.lastname: Martinelli
authors.forename: Giancarlo
authors.initials: G
authors.lastname: Comi
authors.forename: Maria Antonietta
authors.initials: MA
authors.lastname: Volontè
authors.forename: Letizia
authors.initials: L
authors.lastname: Leocani
mesh_terms: D001921:Brain / Q000503:physiopathology*; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology; D005260:Female; D007839:Functional Laterality / Q000502:physiology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D009068:Movement / Q000502:physiology; D010300:Parkinson Disease / Q000503:physiopathology*; D050781:Transcranial Magnetic Stimulation
keywords: Cortical excitability; Interhemispheric balance; Mirror movements; Parkinson's disease; Transcranial magnetic stimulation